Rabbi Yaakov Weinberg zt”l: Notes on Devarim II: El Kol Yisrael
Rashi brings that Moshe wanted to have all Bnai Yisrael present when he gave them mussar so that no one would be able to say later, ‘Had I been there, I never would have let Moshe get away with saying that’.
What’s the pshat? Did Bnai Yisrael have taynos, or a weakness to Moshe’s argument or didn’t they? If they were legitimate complaints, so why wouldn’t they say it even in Moshe’s presence?
The pshat is that we need something to bring us to the emes or we will find all ways and methods to evade the truth. Something or someone must push us to really face up to the truth.
Otherwise, we fool ourselves with rationalizations that we think are real. When you stand before someone like Moshe who speaks the truth of reality you don’t come up with answers. You see the untruth of any taynas.
This is why Moshe wanted everyone present. ‘Kol Yisrael’ also teaches that Moshe would be speaking to everyone each on their levels and appreciations for Moshe’s words.